Automated riser recoil control system and method

ABSTRACT

In one embodiment, the automated riser recoil control system ( 10 ) includes a plurality of riser tensioners ( 20 ), a vessel heave measurement system ( 210 ), and a control processor ( 70 ) in electrical communication with the heave measurement system ( 210 ) and riser tensioners ( 20 ). Each tensioner ( 20 ) includes a piston travel indicator ( 27 ) which signals the processor ( 70 ). In another embodiment, a method for adjusting at least one of the tension forces (F 1 , F 2 ) applied by the tensioners ( 20 ) to the riser ( 60 ) includes determining piston travel velocity for each riser tensioner ( 20 ), measuring vessel heave velocity, calculating the differences between each of the piston travel velocities and the heave velocity, and adjusting the tension force when some preselected number of the velocity differences exceeds a preselected critical velocity difference. Tension force control is typically effected by throttling at least one fluid flow rate within one or more riser tensioner ( 20 ).

RELATED APPLICATIONS

[0001] This application claims the benefit under Title 35 of the UnitedStates Code § 119(e) of U.S. Provisional Patent Application No.60/204,442, filed May 15, 2000.

TECHNICAL FIELD

[0002] This invention relates generally to a system and method forproviding a motion-compensated drilling rig platform. More particularly,the invention relates to an automated system and method which can beused to control marine riser disconnection events and riser tensionerwireline breaks in conjunction with such a platform.

HISTORY OF RELATED ART

[0003] Drilling operations conducted from a floating vessel require aflexible tensioning system which operates to secure the riser conductorbetween the ocean floor (at the well head) and the rig, or vessel. Thetensioning system acts to reduce the effects of vessel heave withrespect to the riser, control the effects of both planned and unplannedriser disconnect operations, and to mitigate the problems created byunexpected breaks or faults in the riser (hereinafter a “disconnectevent”).

[0004] Riser tensioner devices, which form the heart of the tensioningsystem, have been designed to assist in the management of riserconductors attached to drilling rigs, especially with respect tomovement caused by periodic vessel heave. A series of these tensioners,connected to the riser using cables and sheaves, react to relativemovement between the ocean floor and the vessel by adjusting the cablelength to maintain a relatively constant tension on the riser. Anynumber of tensioners, typically deployed in pairs, may be used tosuspend a single riser from the vessel.

[0005] Unexpected events may occur during offshore drilling operations.These may be realized in the form of tensioner wireline breaks, severestorms, or other circumstances which require the vessel/rig operator toact quickly to adjust the tension applied to the riser. The riser mayalso become disconnected from the wellhead for various reasons.

[0006] The need to respond to an unexpected riser disconnect event, ortensioner wireline break, and manage the recoil tension or “slingshot”effect on the vessel induced thereby, provides the motivation to developan automated system and method to control the movement of individualtensioners. The system and method should operate by managing the tensionapplied to the riser using the cables attached to the riser and theriser tensioners in response to sensing an irregular travel velocityexperienced by one or more of the tensioners, such as may be caused by adisconnect event or tensioner wireline break. Thus, the system andmethod should be simple, robust, and fully automatic, such that systemelements are capable of responding to and continuously managing adisconnect event or tensioner wireline break in an automated fashionmore rapidly and reliably than is possible using human operators.

SUMMARY OF THE INVENTION

[0007] In one embodiment, the automated riser recoil control systemincludes a plurality of riser tensioners, a vessel heave measurementsystem, and a control processor in electrical communication with theheave measurement system and the riser tensioners. Each tensionerincludes a piston travel indicator which provides a piston travel signalto the processor, while the vessel heave measurement system provides aheave velocity signal to the processor.

[0008] The processor monitors each of the piston travel signals alongwith the heave velocity signal so as to be able to determine whether apreselected number of piston travel velocities (determined from thepiston travel signals) exceed the vessel heave velocity by some criticalvelocity difference. For example, if sixteen riser tensioners are usedto suspend the marine riser from the heaving vessel, and at least fourof the tensioners show a piston travel velocity which exceeds the heavevelocity by more than about one foot per second (value is typicallybetween about 4-6 feet/second cable speed or about 1.25 feet/secondtensioner piston velocity), then the processor, which is in controllingcommunication with each one of the riser tensioners, can react bycontrolling the force applied to the riser by controlling the rate offluid flow within one or more of the tensioners.

[0009] Typically, each of the riser tensioners includes an accumulatorchamber (blind end of the tensioner) and a piston bore chamber (rod endside of the tensioner), and the fluid flow is controlled within thepiston bore chamber. To control the fluid flow, an orifice-controlledfluid valve is typically placed in fluid communication with the pistonbore chamber. To further control movement of the tensioner, an airshutoff valve is typically placed in fluid communication with theaccumulator chamber and a bank of high pressure air cylinders. Timersmay be applied to adjust the time within which the orifice-controlledfluid valves and air shutoff valves are closed. Finally, to preventextreme movement of the tensioner, a fluid volume speed control valvemay also act to limit the volumetric rate of fluid flow in the pistonbore chamber upon sensing an extreme fluid flow rate within thetensioner.

[0010] In another embodiment, a method for adjusting at least one of thetension forces applied by the tensioners to the riser includes the stepsof determining the piston travel velocity for each riser tensioner,measuring the heave velocity of the vessel, calculating the velocitydifferences between each of the piston travel velocities and the heavevelocity, and adjusting the tension force after determining that somepreselected number of the velocity differences exceeds a preselectedcritical velocity difference (selected by the operator). Again, controlof the tension force is typically effected by throttling the rate of atleast one fluid flow within one or more of the plurality of risertensioners. Air shutoff valves, orifice-controlled fluid valves, andfluid volume speed control valves are all used as previously described.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] A more complete understanding of the structure and operation ofthe present invention may be had by reference to the following detaileddescription taken in conjunction with the accompanying drawings,wherein:

[0012]FIG. 1 is a planar side view of the automated riser recoil controlsystem of the present invention mounted to a heaving vessel from which amarine riser is suspended;

[0013]FIG. 2 is a close-up perspective view of a typical riser tensioner(in dual form);

[0014]FIG. 3 is a schematic block diagram of the automated riser recoilcontrol system of the present invention; and

[0015]FIG. 4 is a flow chart diagram of the method of the presentinvention.

DETAILED DESCRIPTION OF PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS

[0016] Referring now to FIG. 1, it can be seen that the automated riserrecoil control system (10) of the present invention includes a pluralityof riser tensioners (20) in mechanical communication with a heavingvessel (30) and a marine riser (60). Each one of the tensioners (20)applies a corresponding individual tension force (F1, F2) to the riser(60) under heaving conditions, as the vessel (30) responds to ocean wavemovement. The tension forces (F1, F2) are substantially proportional tothe rate of at least one fluid flow within the tensioner. For a moredetailed view of an individual riser tensioner, as shown in adual-tensioner version, see FIG. 2.

[0017] The individual riser tensioners (20) are substantially equivalentto, or identical to, the cable tensioners disclosed in U.S. Pat. Nos.4,351,261 and/or 4,638,978 (incorporated herein by reference in theirentirety). Each riser tensioner (20) may also be similar to or identicalto each of the tensioners that make up the dual tensioner depicted inFIG. 2, which may be purchased from Retsco International, L.P. as RetscoPart No. 112552.

[0018] As can be seen more clearly in FIG. 2, each riser tensioner (20)includes a tensioner piston travel indicator (27) which may be awireline encoder that supplies a distance travel signal for the pistonwithin the tensioner (20). The travel indicator (27) may also take theform of a velocity measurement device, or an acceleration measurementdevice. In any event, the travel indicator (27) provides a signal whichindicates the travel of the piston within the tensioner (20) as thecable (40) moves in reaved engagement with the sheaves (50) and theriser (60). The riser tensioner (20) typically includes an accumulatorchamber in fluid communication with an air shutoff valve (110) and apiston bore chamber in fluid communication with an orifice-controlledfluid valve (120). To prevent extreme movement of the tensioner piston,a fluid volume speed control valve (130) is often inserted between theorifice-controlled fluid valve (120) and the piston bore chamber of thetensioner (20). The operational details of the speed control valve (130)are more fully described in U.S. patent app. Ser. No. 09/733,227(incorporated herein by reference in its entirety).

[0019] The air shutoff valve (110) may be equivalent to or identical toRetsco International, L.P. Part No. 113045. The orifice-control fluidvalve (120) may be equivalent to or identical to Retsco International,L.P. Part No. 113001. Finally, the fluid volume speed control valve(130) may be equivalent to or identical to Retsco International, L.P.Part No. 113102.

[0020] Thus, as can be seen in FIG. 1, the automated riser recoil system(10) operates to control the tension forces (F1, F2) applied to theriser (60) using the cables (40) in reaved engagement with the sheaves(50) of the tensioners (20), the downturn sheaves (55), and the riser(60).

[0021] Normally, as the vessel (30) heaves up and down in response toocean wave movement, the tensioners (20) respond in a passive fashion byplaying out, or taking up, cable (40) in phase with the movement of thevessel (30). This results in the application of substantially evenforces (F1, F2) to the riser as it is suspended from a vessel (30) andconnected to the wellhead (80).

[0022] However, at times, one or more of the cables (40) will break,causing a substantial imbalance in the tension forces (F1, F2). As theapplied tension force from each tensioner (20) is relatively large(e.g., each tensioner supplies about 100,000 lbs. of force), thetensioner piston subjected to the wireline break will tend to move quiterapidly in reaction to the resulting lack of tension. Moreover, in othercircumstances, the marine riser may become disconnected from thewellhead (80) due to unanticipated causes, or as a planned event (e.g.,it is necessary to move the vessel (30) rapidly away from the drillingsite in order to avoid a severe storm or other events).

[0023] When the control processor (70), in electrical communication witheach one of the tensioner piston travel indicators (27) and the vesselheave measurement system (210), determines that one or more of thetensioners (20) has begun to move in such an uncontrolled fashion, theprocessor (70) begins to take action to control the forces (F1, F2)applied to the riser (60).

[0024] For example, referring now to FIG. 3, it can be seen that eachindividual tensioner (20) supplies a piston travel signal (28) usingcommunication line (26) to the processor (70). Of course, the travelindicator (27) may be replaced by a velocimeter or an accelerometer toprovide velocity and/or acceleration signals (28) directly to theprocessor (70), as described above. Similarly, the heave measurementsystem (210) provides a heave velocity signal (215) to the processor(70). However, there are many sensors and systems available, and knownto those skilled in the art, which can provide distance and/oracceleration signals (215) to the processor (70) from the heavemeasurement system (210), since the vessel heave measurement systemtypically includes one or more tri-axial accelerometers and a bi-axistilt sensor coupled to a processor which calculates heave, pitch androll of the vessel. Thus, after a piston distance travel signal (orpiston velocity signal, or piston acceleration signal), is received bythe processor (70), it is converted to a velocity signal (as needed) andcompared with the velocity signal (215) provided by the heavemeasurement system (210). Of course, in a similar fashion, the heavemeasurement system (210) may provide a distance signal or accelerationsignal, which may be converted into a velocity signal, as needed. Theprocessor (70), in turn, is thus in electrical communication with eachone of the tensioner piston travel indicators (27) and the vessel heavemeasurement system (210) and is thereby enabled to monitor each of thepiston travel signals (28) and the heave velocity signal (215).

[0025] It should be noted that numerous other control and communicationsignal lines (29, 179 and 181) can be used to place the processor (70)in controlling communication (i.e., electrical, mechanical, hydraulic,or some combination of these) with any number of other tensioners (20′).Thus, for example, the tensioner (20′) can supply a piston travel signalto the processor (70) using the signal line (181). The tensioner (20′)may, in turn, be controlled by the processor (70) using the air shutoffcontrol valve signal line (179) and the orifice-controlled fluid valvesignal line (181). Any number of tensioners (20, 20′) can be placed incontrolling communication with the processor (70) in this fashion.

[0026] Therefore, when the velocity of the piston (100) within thetensioner (20) exceeds the velocity measured by the heave measurementsystem (210) by some preselected critical velocity difference (e.g., thecritical value is typically selected by the operator to be between about4-6 feet/second of cable (40) speed or about 1.25 feet/second pistonvelocity), the processor (70) can operate to control the fluid (24) flowwithin the tensioner (20), typically using the orifice-controlled fluidvalve (120) to control the fluid flow (24) within the piston borechamber (23). The processor (70) may also operate to control the airshutoff valve (110), which controls the flow of air from the bank ofcylinders (140) and the accumulator chamber (25) of the tensioner (20).

[0027] For example, the processor (70) may send a throttling signal(178) to the orifice-control fluid valve (120) to adjust the valve (120)opening, which regulates the flow of fluid from the accumulator (160)into and out of the piston bore chamber (23). For additionalflexibility, a delay timer (180) can be used to delay the onset of valveclosure for the valve (120) from the time that the signal (178) isasserted by the processor (70). Similarly, the processor (70) may send asignal (177) to the air shutoff valve (110) to isolate the accumulatorchamber (25) within the tensioner (20) from the air bank (140). Again,for additional flexibility, a delay timer (170) may be inserted into thecommunication line between the processor (70) and the valve (110) so asto delay the onset of the air valve (110) closure from the time thesignal (177) is asserted. For reference purposes, the signals (177′,178′) represent delayed signals (177, 178) respectively. Although notshown in FIG. 3, additional timers may also be inserted into thecommunication lines (179, 181). The timer delay periods can be zero, orany other value selected by the system (10) operator.

[0028] Turning now to FIG. 4, the method for adjusting at least onetension force (F1) selected from the plurality of tension forces (F1,F2) applied by the tensioners (20) to the marine riser (60) can be seen.The method begins at step (400) with determining the piston travelvelocities for all of the tensioners (20) used to suspend the riser (50)from the vessel (30). As mentioned above, this typically occurs afterreceiving the piston travel signals supplied from the indicator (27)attached to each of the tensioners (20). The method continues in step(410) with measuring the heave velocity experienced by the heavingvessel (30) as it reacts to wave motion. The heave velocity is typicallydetermined by the processor (70) using the signal supplied from theheave measurement system (210), which indicates the heave velocity ofthe vessel (30).

[0029] The method then continues by calculating a plurality of velocitydifferences, wherein each one of the velocity differences corresponds tothe difference between a selected one of the piston travel velocitiesand the heave velocity. This occurs in step (420). Finally, if aselected number of velocity differences (determined in step (420))exceeds a preselected critical velocity difference (typically selectedby the operator), as determined in step (430), then the tension forceapplied by one or more of the tensioners (20) is adjusted. This occursin step (440).

[0030] The tension force (F1) may be adjusted by throttling the rate ofthe fluid flow within the tensioner using the orifice-controlled fluidvalve (120) (step 450), controlling the air flow within the tensioneraccumulator chamber using the air shutoff valve (110) (step 460), orcontrolling the volumetric rate of flow within the tensioner using thefluid volume speed control valve (130) (step 470). While the air shutoffvalves (110) are typically completely open or completely closed, theorifice-controlled fluid valves (120) are typically set to a preselectedflow limit value in the static condition (e.g., 50% of the maximumvalue), and are modulated to some selected flow rate between about 10%to about 95%, and most preferably to about 15% of the maximum flow ratepermitted by the fully-opened valves (120). As noted above, timers (170,180) can be inserted into the valve control lines for each of thetensioners (20) to delay the application of valve closure/throttlingsignals from the processor (70) to each selected tensioner (20). Thus, atimer (170) can be used to delay closure of the air shutoff valve (110)for a preselected delay time after the processor (70) has determinedthat the preselected number of velocity differences calculated in step(420) exceed the preselected critical velocity difference. Similarly,the timer (180) may be used to delay closure or throttling of theorifice-controlled fluid valve (120) for a preselected time period afterdetermining that a preselected number of the velocity differencescalculated in step (420) exceeds a preselected critical velocitydifference.

[0031] The tension force (F1) applied by a tensioner (20) can thus beadjusted in a number of ways. The most common is by throttling the rateof at least one fluid flow within the selected tensioners. As mentionedabove, this usually occurs by closing orifice-controlled fluid valvesand air shutoff valves. In addition, for extreme piston movementconditions, the fluid volume speed control valve may operateindependently, which acts to limit the volumetric rate of fluid flow inthe tensioner piston bore chamber. The fluid volume speed control valveis typically not operated by the processor (70), but reacts to sensing apredetermined volumetric rate of flow which exceeds a predeterminedcritical volumetric rate of flow, as may be selected by the designer ofthe fluid volume speed control valve. Throughout this document, “fluid”may be considered to be air, oil, water, or any other substantiallynon-solid medium which is used to control movement of the tensioners.

[0032] The processor (70) is in electrical communication with thetensioner piston travel indicators (27) and the heave measurement system(210), and is thus able to continuously or discretely (at periodic oraperiodic intervals) determine the velocity of each individual risertensioner piston (100) and that of the heaving vessel (30). Theprocessor (70) adjusts the tension force applied by each tensioner (20)by controlling the rate of at least one fluid flow within eachtensioner.

[0033] Numerous substitutions and modifications can be made to thesystem (10) as will be recognized by those skilled in the art. Forexample, the processor can be a microprocessor with a memory and programmodule, computer work station, a programmable logic controller, anembedded processor, a signal processor, or any other means capable ofreceiving the distance/velocity/acceleration signals provided by thetensioner piston travel indicators and the heave measurement system, andderiving velocities therefrom (if velocity is not directly supplied).The processor (70) must also be capable of calculating velocitydifferences between each of the pistons traveling within the risertensioners, and the vessel heave velocity; comparing the velocitydifferences to a single critical velocity difference; counting thenumber of velocity differences which exceed the single critical velocitydifference (for comparison to the preselected limit number); andcommanding a preselected number of riser tensioners to adjust theirindividual tension forces applied to the riser.

[0034] Although preferred embodiments of the method and apparatus of thepresent invention have been illustrated in the accompanying Drawings anddescribed in the foregoing Detailed Description, it will be understoodthat the invention is not limited to the embodiments disclosed, but iscapable to numerous rearrangements, modifications and substitutionswithout departing from the scope of the invention as set forth anddefined by the following claims.

What is claimed is:
 1. An automated riser recoil control system, whereinthe riser is suspended from a heaving vessel having a heave velocity,comprising: a plurality of riser tensioners in mechanical communicationwith the vessel and the riser, wherein each one of said plurality ofriser tensioners applies a corresponding individual tension force to theriser under heaving conditions, and wherein each one of thecorresponding individual tension forces is substantially proportional toa rate of at least one fluid flow within a corresponding tensioner, andwherein each one of the corresponding tensioners includes a tensionerpiston travel indicator adapted to provide a piston travel signal; avessel heave measurement system for measuring the heave velocity; and aprocessor in electrical communication with each one of the tensionerpiston travel indicators and the vessel heave measurement system so asto monitor each one of the piston travel signals and the heave velocitysignal, and in controlling communication with each one of the pluralityof riser tensioners so as to control the rate of the at least one fluidflow within at least one of the plurality of tensioners upon determiningthat a preselected number of piston travel velocities determined fromeach one of the plurality of piston travel signals exceed the heavevelocity by a preselected critical velocity difference.
 2. The automatedriser recoil control system of claim 1, wherein the processor adjusts aselected one of the individual tension forces applied to the marineriser by controlling the rate of the at least one fluid flow within acorresponding tensioner.
 3. The automated riser recoil control system ofclaim 1, wherein at least one of the piston travel signals is a pistondistance travel signal.
 4. The automated riser recoil control system ofclaim 1, wherein at least one of the piston travel signals is a pistonvelocity travel signal.
 5. The automated riser recoil control system ofclaim 1, wherein at least one of the piston travel signals is a pistonacceleration travel signal.
 6. The automated riser recoil control systemof claim 1, wherein at least one of the plurality of riser tensionerscomprises an accumulator chamber and a piston bore chamber, and whereinthe at least one fluid flow within the cable tensioner passes throughthe piston bore chamber.
 7. The automated riser recoil control system ofclaim 1, wherein at least one of the plurality of riser tensionersincludes an air shutoff valve, further comprising a first timer adaptedto delay closure of the air shutoff valve for a preselected first delaytime period after determining that the preselected number of pistontravel velocities determined from each one of the plurality of pistontravel signals exceed the heave velocity by the preselected criticalvelocity difference.
 8. The automated riser recoil control system ofclaim 7, wherein at least one of the plurality of riser tensionersincludes an orifice-controlled fluid valve, further comprising a secondtimer adapted to delay closure of the orifice-controlled fluid valve fora second preselected time period after determining that the preselectednumber of piston travel velocities determined from each one of theplurality of piston travel signals exceed the heave velocity by thepreselected critical velocity difference.
 9. The automated riser recoilcontrol system of claim 1, wherein at least one of the plurality ofriser tensioners includes an orifice-controlled fluid valve, furthercomprising a timer adapted to delay closure of the orifice-controlledfluid valve for a preselected time period after determining that thepreselected number of piston travel velocities determined from each oneof the plurality of piston travel signals exceed the heave velocity bythe preselected critical velocity difference.
 10. The automated riserrecoil control system of claim 1, wherein at least one of the pluralityof riser tensioners includes a fluid volume speed control valve whichacts to limit a volumetric rate of fluid flow in the at least one of theplurality of riser tensioners upon sensing a predetermined volumetricrate of flow in excess of a predetermined critical volumetric rate offlow.
 11. A method for adjusting at least one tension force selectedfrom a plurality of tension forces applied by a corresponding pluralityof riser tensioners to a marine riser suspended from a heaving vessel,comprising the steps of: determining a plurality of piston travelvelocities experienced by the plurality of riser tensioners; measuring aheave velocity experienced by the heaving vessel; calculating aplurality of velocity differences, wherein each one of the plurality ofvelocity differences corresponds to a difference between a selected oneof the plurality of piston travel velocities and the heave velocity; andadjusting the at least one tension force upon determining that apreselected number of the plurality of velocity differences exceed apreselected critical velocity difference.
 12. The method of claim 11,wherein at least one of the plurality of riser tensioners applies acorresponding individual tension force to the riser in proportion to arate of at least one fluid flow within the at least one of the pluralityof tensioners, and wherein the step of adjusting the at least onetension force is accomplished by throttling the rate of at least onefluid flow within the at least one of the plurality of riser tensioners.13. The method of claim 11, wherein at least one of the plurality ofriser tensioners includes a fluid volume speed control valve which actsto limit a volumetric rate of fluid flow in the at least one of theplurality of riser tensioners upon sensing a predetermined volumetricrate of flow in excess of a predetermined critical volumetric rate offlow.
 14. The method of claim 11, wherein at least one of the pluralityof riser tensioners includes an air shutoff valve, and wherein a timerdelays closure of the air shutoff valve for a preselected delay timeperiod after determining that the preselected number of the plurality ofvelocity differences exceed a preselected critical velocity difference.15. The method of claim 14, wherein at least one of the plurality ofriser tensioners includes an orifice-controlled fluid valve, and whereina second timer delays closure of the orifice-controlled fluid valve fora second preselected time period after determining that a preselectednumber of the plurality of velocity differences exceed a preselectedcritical velocity difference.
 16. The method of claim 11, wherein atleast one of the plurality of riser tensioners includes anorifice-controlled fluid valve, and wherein a timer delays closure ofthe orifice-controlled fluid valve for a preselected time period afterdetermining that a preselected number of the plurality of velocitydifferences exceed a preselected critical velocity difference.
 17. Themethod of claim 11, wherein the selected one of the plurality of pistontravel velocities is derived from a piston distance travel signal. 18.The method of claim 11, wherein the selected one of the plurality ofpiston travel velocities is derived from a piston acceleration travelsignal.
 19. The method of claim 11, wherein at least one of the risertensioners comprises an accumulation chamber and a piston bore chamber,and wherein the at least one fluid flow within the cable tensionerpasses through the piston bore chamber.